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Abstract: In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 1011 Ωcm and p-type conductivity within hole concentration up to 1018 cm−3 are already available in diameters up to 1.5-in. [ABSTRACT FROM AUTHOR]

Abstract: In this work, results of structural characterization of high-quality ammonothermal GaN are presented. Besides expected low dislocation density (being of the order of 103 cm−2) the most interesting feature seems perfect flatness of the crystal lattice of studied crystals. Regardless the size of crystals, lattice curvature radius exceeds 100m, whereas better crystals reveal radius of several hundred meters and the best above 1000m. Excellent crystallinity manifests in very narrow X-ray diffraction peaks of full-width at half-maximum (FWHM) values about 16arcsec. [Copyright &y& Elsevier]

Abstract: In this paper we are presenting the excellent structural parameters of truly bulk gallium nitride crystals, which were grown by using the AMMONO-Bulk Method. In the crystals grown using this method a low dislocation density in the order of 5×103 cm−2 is readily attainable. At the same time the lattice of ammonothermally grown crystals is extremely uniform. Regardless of the crystal size, the radius of lattice curvature is higher than 100m, whereas in the best crystals it is higher than 1000m. Exceptional crystallinity is also evident in a very narrow X-ray (0002) rocking curves, with FWHM values of about 17arcsec as measured by a standard Panalytical X’pert high-resolution diffractometer. Such excellent structural parameters of AMMONO-GaN crystals show clearly that truly bulk GaN can be grown by using a scalable method, which can be employed in mass production. The authors are convinced that crystals produced using their method will make a breakthrough in the manufacturing of high-power GaN-based devices. [Copyright &y& Elsevier]

In this paper we review the developments of producing non-polar (i.e. m-plane and a-plane) and semi-polar (i.e. (20.1)-plane) wafers by ammonothermal method. The growth method and polishing results are described. We succeeded in producing 26 mm × 26 mm non- and semi-polar wafers. These wafers possess outstanding structural and optical properties, with threading dislocation density of the order of 104 cm-3. Detailed studies of homoepitaxial layers, as well as AlGaN heterostructures are also presented, showing the potential of studied ammonothermal substrates in the fabrication of optoelectronic devices. [ABSTRACT FROM AUTHOR]

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz-Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures. [ABSTRACT FROM AUTHOR]

We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk GaN crystals grown by the ammonothermal method. We observe a high concentration of Ga vacancy related defects in n-type samples with varying free electron and oxygen content. The positron lifetimes found in these samples suggest that the Ga vacancies are complexed with hydrogen impurities. The number of hydrogen atoms in each vacancy decreases with increasing free electron concentration and oxygen and hydrogen content. The local vibrational modes observed in infrared absorption support this conclusion. [ABSTRACT FROM AUTHOR]

Transparency of truly bulk GaN substrates obtained by ammonothermal method was measured in the mid infrared spectral range by Fourier spectroscopy. The same measurements were performed for GaN templates grown by meatlorganic vapour phase epitaxy on sapphire and SiC substrates. It has been clearly observed that truly bulk GaN substrates are transparent up to ∼7 µm whereas GaN templates grown on sapphire and SiC are transparent only up to ∼6.5 and ∼5.5 µm, respectively, due to non-transparency of sapphire and SiC in this spectral range. It has been shown that the transparency 'cut off' at ∼7 µm for GaN crystals results from the absorption of light by the second harmonic of optical phonons, which is very significant due to the strong electron-phonon coupling in this material. Also it has been clearly presented that the absorption 'cut-off' in the infrared spectral region can be easily tuned by increasing of free carrier concentration in GaN. It was observed that the infrared transparency can be shifted from ∼7 to ∼2 µm or even shorter wavelengths when the electron concentration is increased up to ∼1019 cm-3. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: GaN epilayers were grown by metalorganic chemical vapor deposition (MOCVD) on c-, m-, a-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. The influence of (i) the surface preparation of substrates, (ii) MOCVD growth parameters, and (iii) the crystallographic orientation of substrates on the structural and optical properties of GaN epilayers was investigated and carefully analyzed. It was observed that the polishing of substrates and their misorientation have strong impact on the quality of GaN epilayers grown on these substrates. The MOCVD growth process was optimized for epilayers grown on m-plane GaN substrates. The best structural and optical properties were achieved for epilayers deposited at 1075°C and the total reactor pressure of 50mbar. These conditions were applied to grow GaN epilayers on substrates with other (c-, a-, and (20.1)-plane) crystallographic orientations in the same MOCVD process. Particularly good optical properties were obtained for GaN epilayers deposited on polar and non-polar (m- and a-plane) substrates, whereas slightly worse optical properties were observed for epilayers deposited on the semi-polar substrate. It therefore means that MOCVD growth conditions optimized for a given crystallographic direction (m-plane direction in this case) work rather well also for other crystallographic directions. [Copyright &y& Elsevier]